Solid state technology has progressed very far in recent decades. However there is still significant room for improvement in the green/yellow/amber range of the electromagnetic spectrum, approximately 520-600 nm. Direct light emitting diodes (LEDs) or laser diodes in this range are traditionally very low powered and/or inefficient from an electro-optical perspective. This issue with solid state lighting producing light at wavelengths in the green/yellow/amber range is fundamentally limited by the physics of the semiconductor materials used in the construction of these devices, as the band-gap of the materials does not favor emission of light in this spectral range. This problem is commonly referred to in the industry as the ‘green gap’.
Solid state lighting solutions are sought after in all areas of general lighting to improve energy efficiency and increase luminaire lifetime. However, existing solid state apparatuses and methods for generating green light that meets optical power requirements for fluorescence imaging applications are generally costly. The conventional approach has been either devices having low output in the green-yellow band or low coupling efficiency for large area LEDs. LED array or laser pumped crystal or phosphor solutions are relatively expensive compared to arc lamps for some applications with low cost requirements.
A conventional approach to producing broadband light, such as white, is to use ultra violet light, royal blue, or near-ultra violet light from LEDs which have a phosphor powder deposited onto the LED surface. The most popular of these methods is creation of a ‘white’ LED which includes a phosphor of Ce:YAG (cerium doped yttrium aluminum garnet, Y3Al5O12:Ce3+) suspended in an encapsulating material such as silicone, embedded in a transmissive ceramic, or deposited directly onto a blue LED die or die array with a peak wavelength between about 445 nm and 475 nm. The light absorbed by the phosphor is converted to a broadband green/yellow/amber light, which combines with un-absorbed scattered blue light to produce a spectrum that appears white. The brightness of white light is limited by the blue light intensity from the LED, phosphor quantum efficiency, and thermal quenching, especially in the yellow (approximately 560 nm) and amber (approximately 590 nm) spectral bands. Higher power LEDs are available, but the increase in power scales with an increase in the LED emitting area. The coupling efficiency from the illumination source (LED surface) to the objective plane of a microscope objective is inversely proportional to the source size at the same light intensity. Thus, the power delivered to the microscope objective plane cannot typically be increased by simply increasing the LED surface area (it is an Etendue limited optical system).
Another way to achieve bright yellow and amber light is using single crystal Ce:YAG LED pumped by an LED array. The efficiency of such a device is limited by the total internal reflection of such a luminescent material due to its high index of refraction, and more importantly, coupling from LED to crystal. This results in a need of a large number of LEDs to achieve the brightness needed, which increases cost, size, and thermal/electrical requirements on systems employing this method. For example, see U.S. patent Ser. No. 12/187,356.
A third way of generating high powers of light in the green gap consists of using Ce:YAG in crystal form and pumping this structure with a blue (approximately 440-490 nm) laser from the front or back of the crystal. For example, U.S. patent application Ser. No. 13/900,089 describes an optical system using this concept and the predicted improvement in electro-optical coupling efficiency to the focal plane of a microscope has been validated. This approach is very effective in producing a scalable amount of power in the green/yellow band but can be costly. The particular shape and size of the crystal, multiple laser diodes, and cooling methods lead to increased assembly and manufacturing costs.
Other combinations of lasers and phosphors have also been suggested for many high brightness applications including fluorescence illumination, such as U.S. patent application Ser. No. 13/897,237, and other applications such as automotive headlights, for example, U.S. patent application Ser. No. 13/697,782, and digital projection systems, such as U.S. patent application Ser. No. 13/942,603, but these methods are still unable to meet the same cost targets as mercury or xenon arc lamps, which are currently the industry standard. Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings.